So that depends on the version you're playing. If you're playing the public version, you'll have to wait for the next update as that is currently Patreon exclusive. It should be out in the next couple of days (1-2 days). If you have the same update as your friend, make sure that the way you defeated Tank is by using the tease option in battle. The position he ends up in is the position he will be during the scene. If you beat him with attacks then sadly this will be locked and you'll have to start another game, which isn't actually that bad.
I'm pretty sure I defeated him using Tease, but when I get to the cove, I get the option to enter the cave with him. I do, and both north and south exits say WIP and I can't go any further. I go back to the beach and he just says about going back to his boss, yet I'm able to have sex scenes with him easily at the village, no problem. Also, if I do have to start a new game, I'll scream, lol. I'm 150 days into the game with all quests and events complete, bar a few that are still being done, such as the Northcrest Conspiracy and Ratkin stuff.
Then the only thing I can think of is that you don’t have the 0.37 update. As that is the update for the Tank variant scene. The best way to tell is to see what is shown on the title screen. Should be big and bold but in case you don’t see it, 0.36 should have Logan on the main title screen while 0.37 should have Caleb.
Well good news, in 0.38 you can now set Tank behavior by giving him potions so hopefully that may fix your issue and you get the see the various scenes for Tank. You need Dominant and/or Submissive potions along with a Top or Bottom potion, with that you can change his position when you talk with him. All combinations are available minus the Dominant Bottom, that hasn't been written yet.